This invention relates to pulse duration modulation and, more particularly, to a method and apparatus for providing a pulse duration modulation signal by using digital techniques useful for transmitters operating in a switched mode.
Pulse duration modulation (PDM) codes a relatively low frequency signal into a two-state waveform. Advantages exist in using this binary (two-state) time series. One consequence is that an amplifier that processes the coded waveform operates in a "switch" (Class D) mode instead of a "linear" mode (Class A or B). In other words, Class D amplifiers respond to PDM commands to turn solid state devices either "ON" or "OFF" in proper sequence. The output stage of the power transmitter, controlled in such a manner, amplifies the two-state waveform.
An important characteristic of the subsequent high power output signal is its low frequency content. If the PDM output is low-pass filtered, low frequency spectral components track linearly with the original low frequency input over a broad dynamic range. The input signal (the low frequency signal) is modulated by a high frequency carrier in the pulsewidths of the switch mode wave.
Several methods of generating a pulse duration modulated signal are available. A PDM waveform can be formed in both an analog or digital way. Digital processing is advantageous for several reasons. Firstly, if the input waveform is already in a digital format, digital-to-analog conversion is not required. Another important factor is that reliability increases by using a digital approach, since problems inherent to analog circuits do not exist in digital hardware. In the digital domain, however, other factors must be considered if a truly analogous approach to analog PDM is taken.
A prior art analog (continuous time) circuit 10 for generating a pulse duration modulated signal is shown in FIG. 1. X(t) and D(t) are both continuous waveforms. X(t) is the low frequency input signal. D(t) is a relatively high frequency triangle signal which will be referred to as a dither waveform. The amplitudes of the two signals are compared by a linear comparator 11 to provide E(t) which, in turn, drives a two state device 12 such as a relay whose output terminal is alternately connected to voltage sources +G, -G to provide an output signal Y(t). If X(t) is greater than D(t), Y(t) having a level +G is generated at the relay's output. The opposite output level, -G, is produced if X(t) is less than D(t).
Analog circuitry has inherent problems that are difficult, if not impossible, to overcome. Components must be properly calibrated to compensate for hardware tolerances and drift. For these reasons, processing inaccuracies are induced due to physical device limitations. Circuit input must also be considered. A digital input to an analog PDM system requires D/A conversion of the signal prior to modulation to produce the binary output waveform. The additional hardware complexity in the D/A conversion process may reduce system reliability.
An analogous pulse duration modulator can be built in the digital domain. An approach to this circuit design is to compare, simultaneously, at a fixed rate, samples of a digital input signal X(nt) with coincident samples of a digital triangular wave D(nt). A very high input data rate is needed to compare X(nt) and D(nt) to obtain appropriate PDM waveform definition. As a consequence, high speed digital hardware is required to compare the two signals and to sufficiently define the PDM waveform. Hardware presently is not available to construct an analogous digital amplifier that will meet needed output requirements.